1. Field of the Invention
This invention relates, in general, to optical printheads and, more specifically, to LED printheads for use in copiers, duplicators, printers, and like devices which produce hard copies.
2. Description of the Prior Art
Optical printheads are used in copiers, duplicators, and printers to expose a photoconductive surface or film in the apparatus in such a manner that a latent image is formed on the film. The image is then developed and transferred to paper for producing a hard copy output from the apparatus. Many optical printheads use light emitting diode (LED) arrays to generate the radiation or light necessary to expose the photoconductive film. The LED arrays must be supported by other components or structures to make a working printhead, such as driver IC's, interconnecting boards, heat sinks, focusing lenses, and structural mounting supports. Because of the tight tolerances required for the alignment and spacings of the LED's to produce high quality copies, realignment is often necessary when service on or replacements of the printhead are required.
Another important aspect of printhead construction is the ability of the structure to protect and regulate the environment of the delicate components within the printhead. It is desirable to keep dust and other contamination off of the LED's in order to maintain the efficiency of the LED light sources. In addition, it is desirable to keep contaminants away from the very fine interconnecting wires used to connect together the various chips, circuits, and boards to make a complete working system. According to the prior art, a separate transparent cover is usually used between the lens and the LED boards to protect and seal the LED's. While this may offer satisfactory contaminant protection, the loss in light intensity by passing through the lens can reduce the efficiency of the printhead light source, and is just another component which may need periodic attention and cleaning.
Occasionally, printheads must be changed because of a malfunction or the availability of a better performing printhead. Traditionally, changing an LED printhead requires a certain amount of alignment to properly focus the light on the surface of the film. This is usually necessitated by the fact that the printhead contains less than the complete inventory of components necessary to produce and focus the light onto the film. For example, printheads without integral lenses must be readjusted because the spacing between the lens and the LED's changes when the printhead is replaced. In addition, the mounting of conventional printheads is sometimes very time consuming.
Therefore, it is desirable, and it is an object of this invention, to provide an LED printhead which conveniently protects the delicate portions of the printhead from dust and contaminants without the need for a separate light absorbing protective glass cover. It is also desirable, and another object of this invention, to provide a modular printhead which can be easily removed and replaced without the need to make adjustments to the alignment of the optical path between the LED's and the photoconductive film, including the focusing lens.
U.S. Pat. Nos. 4,715,682 and 4,728,981, both issued to the assignee of the present invention, disclose optical printheads which are representative of the current state of the art. These printheads include a separate glass cover over the LED array in addition to the lens system. The mounting supports for the printheads require alignment of the printhead after it has been installed in the copier or printer. Removing the printhead destroys the exact alignment, and replacement by another printhead requires new alignment. U.S. Pat. No. 4,750,010, also assigned to the assignee of the present invention, discloses circuitry for controlling and driving the LED's in optical printheads, and may be referred to for additional background information relating to LED printheads.